Surface Flux Baseline 92 Derived (FIFE)

Summary:

The FIFE Surface Flux Baseline 92 Derived Data Set was compiled from the original surface flux data collected during FIFE (i.e., no measurements were made specifically for this data set). This data set contains data collected from mid-May through mid-October, 1987 at 21 stations located within 19 sitegrids spread throughout the FIFE study area. For a description of the theory behind the original surface flux measurements see the documentation for each of the original surface flux data sets.

Surface heat flux data routinely have erroneous jumps (i.e., spikes) in the latent and sensible heat flux time series in the early morning and evening hours due to small gradients in the measured data. A series of tests were developed to identify these spikes and flag them. Flux data obtained from Bowen ratio sites are also checked for energy imbalances. These data were also compared to model results. The consistency between these two methods is indicated in this data set.

1. Data Set Overview:

Data Set Identification:

Data Set Introduction:

The FIFE Surface Flux Baseline 92 Derived Data Set was compiled from the original surface flux data collected during FIFE (i.e., no measurements were made specifically for this data set). This data set contains data collected from mid-May through mid-October, 1987 at 21 stations located within 19 sitegrids spread throughout the FIFE study area.

Objective/Purpose:

A standardized Surface Flux Baseline data set was produced to replace the Surface Flux Baseline July 1990 data set. In the data set described here 'spikes' and energy imbalances in the surface flux time series are flagged. In addition a model-based check on the reasonableness of the fluxes is provided.

Summary of Parameters:

Discussion:

Surface heat flux data routinely have erroneous jumps (i.e. spikes) in the latent and sensible heat flux time series in the early morning and evening hours due to small gradients in the measured data. A series of tests were developed to identify these spikes and flag them. Flux data obtained from Bowen ratio sites are also checked for energy imbalances. These data were also compared to model results. The consistency between these two methods is indicated in this data set. These data were collected from mid-May through mid-October, 1987 at 21 stations located within 19 sitegrids spread throughout the FIFE study area. During this time period measurements were made almost every day.

Title of Investigation:

Contact Information:

Requested Form of Acknowledgment.

The Surface Flux Baseline 1992 data were compiled at NASA Goddard Space Flight Center under the direction of Dr. P. Sellers. The dedicated effort of M. Heiser in the compilation of these data is particularly appreciated.

3. Theory of Measurements:

These data are compiled from the original surface flux data collected during FIFE. Consequently, no measurements were made specifically for this data set. For a description of the theory behind the original surface flux measurements see the documentation for each of the original surface data sets listed in the Related Data Sets Section above.

4. Equipment:

Sensor/Instrument Description:

A desktop computer was used to identify and flag the 'spikes' in the data. In addition, this computer was used to compute the model results which were compared to the original, field-measured fluxes.

Principles of Operation:

Sensor/Instrument Measurement Geometry:

Manufacturer of Sensor/Instrument:

Not available at this revision.

Calibration:

Calibration is not applicable to this data set since the data are compiled from the original surface flux data. A description of the calibration procedures used for each of these underlying data sets can be found in the individual documentation for the Bowen ratio and Eddy correlation surface flux data sets (see the Related Data Sets Section for a list of these data sets).

Tolerance:

Frequency of Calibration:

Other Calibration Information:

5. Data Acquisition Methods:

The surface flux data were acquired from the FIFE Information System. These data are available on FIFE CD-ROM Volume 1 in the DATA\SUR_FLUX\30_MIN\ subdirectory.

6. Observations:

Data Notes:

Not available.

Field Notes:

Not applicable.

7. Data Description:

Spatial Characteristics:

The FIFE study area, with areal extent of 15 km by 15 km, is located south of the Tuttle Reservoir and Kansas River, and about 10 km from Manhattan, Kansas, USA. The northwest corner of the area has UTM coordinates of 4,334,000 Northing and 705,000 Easting in UTM Zone 14.

Spatial Coverage:

Data were compiled from 21 surface flux stations located in 19 sitegrids scattered throughout the FIFE study area.

Spatial Coverage Map:

Spatial Resolution:

The surface fluxes for each station are representative of an area extending approximately 100 meters upwind from the surface flux equipment.

Projection:

Not available.

Grid Description:

Not available.

Temporal Characteristics:

Temporal Coverage:

Surface fluxes were measured from May 26 to October 16 in 1987.

Temporal Coverage Map:

Not available.

Temporal Resolution:

Surface fluxes were reported at 30 minute intervals for all sites except for site 8739-ECB (site 26) which reported the data at 1 hour intervals.

Data Characteristics:

The SQL definition for this table is found in the SF_BL92.TDF file located on FIFE CD-ROM Volume 1.

Parameter/Variable Name Parameter/Variable Description Range Units Source
SITEGRID_ID
This is a FIS grid location code.
Site grid codes (SSEE-III) give
the south (SS) and the east (EE)
cell number in a 100 x 100 array
of 200 m square cells. The last 3
characters (III) are an instrument
identifier.
STATION_ID
The station ID designating the
location of the observations.
OBS_DATE
The date of the observations,
in the format (DD-mmm-YY).
OBS_TIME
The time that the observation [GMT]
was taken in GMT. The format is
(HHMM).
NET_RADTN
The Net Radiation. ** [Watts]
[meter^-2]
LATENT_HEAT_FLUX
The Latent Heat flux. ** [Watts]
[meter^-2]
SENSIBLE_HEAT_FLUX
The Sensible Heat flux. ** [Watts]
[meter^-2]
SOIL_HEAT_FLUX
The Surface Soil Heat flux. ** [Watts]
[meter^-2]
SPIKE_FLAG
A flag indicating if this data
is considered "spike" (or glich)
data. 0 means good data, 1 means
spike data, 2 means available
energy does not balance, 3 means
both 1 and 2.
IDF_FLAG
A flag showing the reasonableness
of the data (lower is better)
based on comparisons with model
results. See the
Processing Steps Section
of the document for the
meaning of the values.
FIFE_DATA_CRTFCN_CODE
The FIFE Certification Code for *
the data, in the following format:
CPI (Certified by PI), CPI-???
(CPI - questionable data).
LAST_REVISION_DATE
in the format (DD-MMM-YY).

Footnotes:

* Valid levels

The primary certification codes are:

EXM Example or Test data (not for release) PRE Preliminary (unchecked, use at your own risk) CPI Checked by Principal Investigator (reviewed for quality) CGR Checked by a group and reconciled (data comparisons and cross checks)

The certification code modifiers are:

PRE-NFP Preliminary - Not for publication, at the request of investigator. CPI-MRG PAMS data that is "merged" from two separate receiving stations to eliminate transmission errors. CPI-??? Investigator thinks data item may be questionable.

8. Data Organization:

Data Granularity:

The surface fluxes for each station are representative of an area extending approximately 100 meters upwind from the surface flux equipment. Surface fluxes were reported at 30 minute intervals for all sites except for site 8739-ECB (site 26) which reported the data at 1 hour intervals.

A general description of data granularity as it applies to the IMS appears in the EOSDIS Glossary.

Data Format:

The CD-ROM file format consists of numerical and character fields of varying length separated by commas. The character fields are enclosed with a single apostrophe. There are no spaces between the fields. Each file begins with five header records. Header records contain the following information:

Record 1 Name of this file, its table name, number of records in this file, path and name of the document that describes the data in this file, and name of principal investigator for these data. Record 2 Path and filename of the previous data set, and path and filename of the next data set. (Path and filenames for files that contain another set of data taken at the same site on the same day.) Record 3 Path and filename of the previous site, and path and filename of the next site. (Path and filenames for files of the same data set taken on the same day for the previous and next sites (sequentially numbered by SITEGRID_ID)). Record 4 Path and filename of the previous date, and path and filename of the next date. (Path and filenames for files of the same data set taken at the same site for the previous and next date.) Record 5 Column names for the data within the file, delimited by commas. Record 6 Data records begin. Each field represents one of the attributes listed in the chart in the Data Characteristics Section and described in detail in the TDF file. These fields are in the same order as in the chart.

9. Data Manipulations:

Formulae:

Derivation Techniques and Algorithms:

Variable Definitions:
RN = net radiation [watts][m^-2]

GH = ground heat flux [watts][m^-2]

LE = latent heat flux [watts][m^-2]

SH = sensible heat flux [watts][m^-2]

m = meters

Available Energy Consistency Check
DELTA = (RN - GH) - (LE + SH)

At a given time, Bowen ratio stations should have equal available
energies since RN and GH are used to calculate LE and SH. Flux
data for Bowen ratio stations will fail the test if the absolute
value of DELTA is greater than 1 [watts][m^-2]. Since eddy correlation
stations measure LE and SH directly with RN and GH measured by
other instruments, one would not expect the available energies
to be equal. If they are equal it is assumed that the GH is computed
as a residual. Therefore, if the absolute value of DELTA is less
than 1 [watts][m^-2], GH is set to an undefined value.

GH Smoothness Test

Spikes have been observed in some ground heat flux data records.
This can result in incorrect values for latent and sensible heat
fluxes at Bowen ratio sites due to the manner in which these fluxes
are derived. These points are flagged as being a spike when the
absolute value of the rate of change of the ground heat

flux exceeds 1.5 times the rate of change of the net radiation
or 50.0 [watts][m^-2][hour^-1] (whichever is greater). This test
is applied only to Bowen Ratio stations when the net radiation
exceeds 100 [watts][m^-2] and must be true for both the time period
leading up to and immediately following the time of the observation.

Spike Identification Test

As an initial spike identification test, the following conditions
must be satisfied. The sensible heat flux must be either less
than or greater than the values reported before and after the
time in question.

AND

The latent heat flux must be either less than or greater than
the values reported before and after the time in question.

2nd Derivative Test

The 2nd derivatives with respect to time of the latent and sensible
heat fluxes must have opposite signs for a point to be a spike.

Absolute Departure Test
LIM1 = (LE + SH) / 4.0

Both the sensible and latent heat fluxes must exceed the average
of the observations immediately preceding and following the flux
observation time by LIM1 or 25 [watts][m^-2] (whichever is greater)
for a point to be a spike. This test forces spike checking on
points that deviate from the mean by at least 25 [watts][m^-2]
thus preserving small variations in the flux time series that
otherwise might be eliminated.

Heat Flux Jump Test

In the early morning and evening hours, there are sometimes large
absolute differences between the sensible and latent heat fluxes
that are unrealistic. These points are eliminated if the absolute
value of the difference between the sensible and latent heat flux
exceeds 100 [watts][m^-2]. This test is only applied when the
sum of the latent and sensible heat flux is less than 25 [watts][m^-2].

Isolated Point Test

If both the latent and sensible heat fluxes preceding and following
an observation time are undefined or flagged as a spike, the latent
and sensible heat flux at the observation time is considered unreliable
and flagged as a spike.

This test for spikes is applied to the data for each observation
time except for the first and last observations since the filter
requires data preceding and following the time of the observation.
The tests are run beginning with the second available observation
time and ending with the next to last available observation time.
both latent and sensible heat fluxes must be present at the time
of the observation as well as at the immediately preceding or
following times for these tests to be applied. If there are any
absent data, the program moves on to the next observation time.
(see the Derivation Techniques and Algorithms Section.

For each observation apply:

Ground heat flux smoothness test If the ground heat flux is
not smooth, the data is considered unreliable, flagged as a spike
and then the program moves on to the next observation.

Spike Identification Test

LE, SH 2nd Derivative Test

LE, SH Absolute Departure Test

The data must pass all three of these tests for it to be flagged as a spike.

Since this filter requires data preceding and following the time of the observation, these tests are run beginning with the second available observation time and ending with the next to last available observation time (see the Derivation Techniques and Algorithms Section.

A plot of the net radiation measured at each flux site against the site averaged Q*6 net radiation revealed that a regression using two linear functions would do the best job at adjusting the net radiation. One function describes the adjustment when RN is between 0 and YRN and the second function describes the adjustment when RN is greater than YRN. There is no adjustment made when RN is less than zero. Using the values in the table below, RN is adjusted as follows:

RN < YRN ..... ARN = (RN - YINT1) / SLOP1

RN > YRN ..... ARN = (RN - YINT2) / SLOP2

Allowing the ground heat flux to keep its original value, corresponding adjustments are then made to the sensible and latent heat fluxes using a ratio of the adjusted available energy to the original available energy.

Calculated Variables:

Graphs and Plots:

10. Errors:

Sources of Error:

Quality Assessment:

It was recognized early in the study that standardization of "constants" (e.g. physical constants of the air, psychrometic constant, etc.), methods of computation, integration and reporting time, etc. were necessary. These were agreed upon in planning sessions. Preliminary data sets were compared among stations and instruments from different manufacturers for estimating net radiation, soil heat flux, water vapor density, temperature, solar radiation, and wind speed, it was necessary to have confidence that differences in observations were due to site differences and not due to instrumentation.

Data Validation by Source:

The hydrological Science Branch at NASA Goddard Space Flight Center was given the responsibility to compare flux data from all flux stations. This served two purposes: 1) as a data quality check, and 2) a preliminary analysis of site differences.

Confidence Level/Accuracy Judgment:

Model dependent flags by station and Intensive Field Campaign

This flag is computed based on computer runs using the Simple Biosphere (SiB) model. It gives an indication of the model's ability to reproduce the observed latent and sensible heat flux time series. A comparison of the canopy conductance computed for each IFC provides another check on the reasonableness of the data. Theory and observations suggest that this quantity should decrease from IFC-2 to IFC-4 as the site dries out. IFC-1 is disregarded since it was a wet period with high soil moisture contributions to the heat flux that would contaminate the calculated values of the canopy conductance.

Contradictions to this expectation are noted. Finally, a high canopy conductance in IFC-4 when the site was dry points to a bias in the data and is noted in the IDF_FLAG descriptor.

SIGH = standard deviation between the observed and calculated
sensible heat flux over a given day

SIGE = standard deviation
between the observed and calculated latent heat flux over a given
day

ABAR = average observed available energy for a given day

SUM2 = SIGH * SIGH + SIGE * SIGE

The error is computed by taking the square root of SUM2 and dividing
that by ABAR.

IDF Flag Definitions

IDF DEFINITION

--- ----------

0 Good data

1 The model did not have a good fit to the observed latent
and sensible heat fluxes for that day. This condition was
considered met when the error was greater than or equal to
0.10 for a particular IFC.

2 The results are contrary to the expectation of decreasing
values of DF through IFC's 2, 3, 4 as suggested by theory
and observations.

3 Error conditions 1 and 2 are met.

4 Abnormally high values of DF in IFC-4 suggest a bias in
the data. Any values of DF in IFC-4 greater than 1.29e-5
(m^3/joules) (the average DF plus one standard deviation)
were considered abnormally high.

Measurement Error for Parameters:

The errors associated with the original flux measurements are described in the documents describing the original data sets. See the documents listed in the Related Data Sets Section for this information.

Additional Quality Assessments:

FIS staff applied a general QA procedure to the data to identify inconsistencies and problems for potential users. As a general procedure, the FIS QA consisted of examining the maximum, minimum, average, and standard deviation for each numerical field in the data table. Inconsistencies and problems found in the QA check are described in the Known Problems with the Data Section.

Data Verification by Data Center:

The data verification performed by the ORNL DAAC deals with the quality of the data format, media, and readability. The ORNL DAAC does not make an assessment of the quality of the data itself except during the course of performing other QA procedures as described below.

The FIFE data were transferred to the ORNL DAAC via CD-ROM. These CD-ROMs are distributed by the ORNL DAAC unmodified as a set or in individual volumes, as requested. In addition, the DAAC has incorporated each of the 98 FIFE tabular datasets from the CD-ROMs into its online data holdings. Incorporation of these data involved the following steps:

Copying the entire FIFE Volume 1, maintaining the directory
structure on the CD-ROM.

Using data files, documentation, and SQL code provided on
the CD-ROM to create a database in Statistical Analysis System
(SAS).

Each distinct type of data (i.e. "data set" on the CD-ROM), is accompanied by a documentation file (i.e., .doc file) and a data format/structure definition file (i.e., .tdf file). The data format files on the CD-ROM are Oracle SQL commands (e.g., "create table") that can be used to set up a relational database table structure. This file provides column/variable names, character/numeric type, length, and format, and labels/comments. These SQL commands were converted to SAS code and were used to create SAS data sets and subsequently to input data files directly from the CD-ROM into a SAS dataset. During this process, file names and directory paths were captured and metadata was extracted to the extent possible electronically. No files were found to be corrupted or unreadable during the conversion process.

Additional Quality Assurance procedures were performed as follows:

Statistical operations were performed to calculate minimum
and maximum values for all numeric fields and to create a listing
of all values of the character fields. During this process, it
was determined that various conventions were used to represent
missing values. (Note: no modifications were made to any data
by the DAAC). In most cases, missing value identification conventions
were discussed in the accompanying .doc file. Based on a visual
check of the minimum and maximum values, no glaring errors or
holes were identified that might indicate errors introduced during
CD-ROM mastering by the FIFE project or data ingest by the DAAC.

Some minor inconsistencies and typographical errors were identified
in some of the character fields and column labels, however, no
modifications were made to the data by the DAAC.

Some conversions of ASCII data were necessary to move the
data from a DOS platform to a UNIX platform. Standard operating
system conversion utilities were used (e.g., dos2unix).

Much of the metadata required for archival is imbedded in
the narrative documentation accompanying the data sets and extracted
manually by DAAC staff who have read the .doc files provided on
the CD-ROM and have hand entered this information into the metadata
database maintained by the DAAC. QA procedures have been performed
on these metadata to identify and eliminate typographical errors
and inconsistencies in naming conventions, to ensure that all
required metadata is present, and to ensure the accuracy of file
names and paths for retrieval.

Data requested for distribution to users are checked to verify
that files copied from disk to other media remain uncorrupted.

As errors are discovered in the online tabular data by investigators, users, or DAAC staff, corrections are made in cooperation with the principal investigators. These corrections are then distributed to users. CD-ROM data are corrected when re-mastering occurs for replenishment of CD-ROM stock.

11. Notes:

Limitations of the Data:

Not available.

Known Problems with the Data:

The following problem were encountered by the FIS staff during their general QA procedures:

Unusually high latent heat flux values of 896 [watts][m^-2]
were reported on October 6, 1987 at station 2 (1916-BRS).

Unusually high sensible heat flux values of greater than 1000
[watts][m^-2], with a maximum of 4368 [watts][m^-2], were reported
on nine occasions from October 2 through October 4, 1987 at station
36 (2655-BRL).

Usage Guidance:

Not available at this revision.

Any Other Relevant Information about the Study:

Not available at this revision.

12. Application of the Data Set:

Not available at this revision.

13. Future Modifications and Plans:

The FIFE field campaigns were held in 1987 and 1989 and there are no plans for new data collection. Field work continues near the FIFE site at the Long-Term Ecological Research (LTER) Network Konza research site (i.e., LTER continues to monitor the site). The FIFE investigators are continuing to analyze and model the data from the field campaigns to produce new data products.

14. Software:

Software to access the data set is available on the all volumes of the FIFE CD-ROM set. For a detailed description of the available software see the Software Description Document.

Data Center Identification:

Procedures for Obtaining Data:

Users may place requests by telephone, electronic mail, or FAX. Data is also available via the World Wide Web at http://daac.ornl.gov.

Data Center Status/Plans:

FIFE data are available from the ORNL DAAC. Please contact the ORNL DAAC User Services Office for the most current information about these data.

16. Output Products and Availability:

The Surface Flux Baseline 1992 data are available on FIFE CD-ROM Volume 1. The CD-ROM filename is as follows:

\DATA\SUR_FLUX\BASEL_92\GRIDxxxx\YyyMmm\ydddgrid.SFB

Where xxxx is the four digit code for the location within the FIFE site grid, yy is the last two digits of the year (e.g. Y87 = 1987), mm is the month of the year (e.g. M12 = December), and ddd is the day of the year, (e.g. 061 = sixty-first day in the year). Note: capital letters indicate fixed values that appear on the CD-ROM exactly as shown here, lower case indicates characters (values) that change for each path and file.

The format used for the filenames is: ydddgrid.sfx, where grid is the four-number code for the location within the FIFE site grid, y is the last digit of the year (e.g. 7 = 1987, and 9 = 1989), and ddd is the day of the year. The filename extension (.sfx), identifies the data set content for the file (see the Data Characteristics Section and is equal to .SFB for this data set.

Archive/DBMS Usage Documentation.

Contact the EOS Distributed Active Archive Center (DAAC) at Oak Ridge National Laboratory (ORNL), Oak Ridge, Tennessee (see the Data Center Identification Section). Documentation about using the archive and/or online access to the data at the ORNL DAAC is not available at this revision.

20. Document Information:

April 26, 1994 (citation revised on October 15, 2002).

Warning: This document has not been checked for technical or editorial accuracy by the FIFE Information Scientist. There may be inconsistencies with other documents, technical or editorial errors that were inadvertently introduced when the document was compiled, or references to preliminary data that were not included on the final CD-ROM. Previous versions of this document have been reviewed by the Principal Investigator, the person who transmitted the data to FIS, a FIS staff member, or a FIFE scientist generally familiar with the data.